In a digital sampling mixer (DSM) samples of a signal are taken at high speed in order to down-convert what is typically a radio frequency (RF) signal to a lower, more manageable rate. Such sampling requires activation of switches at a rate higher than the RF signal itself. For example, in a TV tuner application the highest RF signal may be 1 GHz and the samples taken at 2.4 GHz or higher. Such a high speed clock is inconvenient and costly.
U.S. Pat. No. 7,028,070 relies on a delay line, the output of which activates a series of SHA (sample and hold amplifiers) in sequence. However, an imperfection arises due to the mismatch of delay time in the delay line. This mismatch, being random, executes a classic “random walk” around an ideal graph of sample time vs position in the sequence. Such a classic random walk exhibits a nominal (or expected) deviation of sqrt(N) where N is the length of the sequence. Therefore given a sequence of say 100 delay elements, each element having a delay of, for example 1 nanosecond, and further having a random deviation of say 100 picoseconds, the mean deviation from the ideal linear progression vs. time is sqrt(100)*100 picoseconds=1 nanosecond. But in practice the deviation is well over 10 nanoseconds. In an actual example, a chip with 320 picoseconds delay with 30 picoseconds deviation and 121 elements exhibits a 2 nanosecond deviation, well over the theoretical mean deviation of sqtr(121)*30 picoseconds=330 picoseconds.
The delay line does not exhibit the characteristic of a random walk. The error, the 100 picoseconds in the first example, is correlated along the sequence. For example, the first 50 delay elements are all 0.9 nanoseconds and the last 50 delay elements are all 1.1 nanoseconds. There is still 100 nanoseconds of total delay. However, although the mid point of the delay chain is expected to be at 50 nanoseconds, the mid point of the delay chain is in fact occurring at 0.9 nanoseconds*50=45 nanoseconds, or 5 nanoseconds away from the expected point. That is 5 times the value predicted by the expected random-walk.